Precipitator for an electrostatic filter

ABSTRACT

In an electrostatic precipitator for an electrostatic filter at least one group (11 and/or 12) of electrode or plate elements of like polarity are made of or coated with a semiconducting or dissipative material, and the electrode elements of at least one group (12) include, or are associated with, a screen (16) of an insulating material at the edge thereof which is directed against the stream (A) of air through the precipitator.

BACKGROUND OF THE INVENTION

This invention relates to a precipitator for a two-stage electrostaticfilter.

WO 93/16807 discloses a two-stage electrostatic filter which comprisesan ionizer and a capacitor or electrostatic precipitator (collector)positioned downstream of the ionizer. Advantageously, the electrodeelements of the precipitator, which in the illustrated embodiment areflat plates but may have different shapes in other embodiments, are madeof a material that may be designated as highly resistive or antistatic(so-called dissipative material). With such a material a substantialimprovement can be achieved, because the voltage that can be producedbetween adjacent electrode elements is self-regulating and can reachhigher values than in the customary electrostatic filters havingelectrode elements made of a material of low resistivity, such asaluminium, and having a galvanic connection to the voltage source.

In the embodiment disclosed in the above-mentioned publication, theplates preferably are charged by the ion current from the coronaelectrode of the ionizer, and the voltage between them is stabilizedbecause the electrode elements have field-concentrating formations,which may take the shape of, for example, sharp edges or other pointedparts of the electrode elements.

SUMMARY OF THE INVENTION

The present invention advantageously can be embodied in two-stageelectrostatic filters of the kind disclosed in the above-mentionedpublication, but it is not limited to use in filters of that kind. Forexample, the charged particles by means of which the precipitation ofdust on the electrode elements is brought about need not necessarily beproduced in an ionizer of the type disclosed, but may be produced andcarried to and through the precipitator in any suitable manner.Moreover, it is not necessary that the electrode elements be charged bythe air ions produced in the ionizer. Instead, the required voltagebetween adjacent electrode elements may be maintained by a connection ofthe electrode elements to a high-voltage source, preferably a veryhighly resistive connection.

An object of the invention is to provide a high, and yet stable,voltage, i.e. a high threshold voltage, with narrow air gaps betweenadjacent electrode or plate elements of the precipitator, so that theprecipitator may be used in applications in which the precipitator hasto meet high stringent demands in respect of precipitation capability.

According to the invention, this object is attained by using a highlyresistive, antistatic or almost antistatic (dissipative) material of, orcoating on, the electrode elements of the precipitator and at the sametime avoiding field concentrations, especially at those edges of theelectrode elements which are at the upstream end of the electrodeelements and, consequently, confront the stream of air that carries thecharged particles to be precipitated on the electrode elements.

According to the invention, this can be accomplished by providing theelectrode element edges in question with a screen made of anon-conducting, insulating material. This screen need not necessarily beapplied to the edges, meaning that it need not be physically connectedwith the edges, but may be slightly spaced from them; it is sufficientthat the screen is constructed and positioned such that itelectrostatically screens the edges from the electric field.

The screening may be provided by making the electrode elements from aninsulating material and coating them with a semiconducting or antistaticmaterial such that their edges are left insulated (uncoated) so thatonly the electrode element surface inwardly (downstream) of the edges issemiconducting or antistatic.

The same effect may be achieved if the electrode elements are made froma semiconducting or antistatic material and the edges thereof are coatedwith an insulating, non-conducting material.

Advantageously, the first-mentioned way of providing the screening maybe used in cases in which a cellulose material, such as paper orcardboard, is used for the electrode elements and the precipitator is tooperate in a dry and warm environment. An example of such cases isprecipitators of electrostatic filters for vacuum cleaners. In suchcases, the excellent natural semiconducting or antistatic properties ofthe cellulose material may be lost after some time of operation becauseof the heating of the air that occurs in the vacuum cleaner, so that thematerial becomes comparable to an insulating material.

In the preferred embodiment of the present invention, the electrodeelements may advantageously be charged by air ions supplied from an ioncurrent generated by a corona electrode positioned upstream of theprecipitator in accordance with the disclosure of the above-mentionedpublication.

To this end, the electrode elements to be charged in this manner shouldproject in the upstream direction beyond the electrode elements of adifferent polarity--preferably, the last-mentioned electrode elementsare grounded and for the sake of simplicity will be referred tohereinafter as the grounded electrode elements whereas thefirst-mentioned electrode elements will be referred to as the chargedelectrode elements--and they should also be screened in theabove-explained manner so that field-concentrating formations at theedges are screened. If not screened, these formations would limit thevoltage because of discharges to the grounded electrode elements.

Similar considerations apply to the trailing or down-stream end of thegrounded electrode elements. In the preferred embodiment the groundedelectrode elements extend in the downstream direction beyond the chargedelectrode elements. The trailing edge of the grounded electrodeelements, which thus protrudes in the downstream direction, mayadvantageously also be antistatic or semiconducting so that it canreadily be connected to ground.

In the preferred embodiment of the present invention, the electrodeelements are insulated from one another. The insulation may be arrangedsuch that it covers the electrode element edges extending in thedirection of air flow so that these edges are also screened from theelectrical field.

As is apparent from this description, the invention comprises screeningof the field-concentrating formations which in the electrostatic filteraccording to the above-mentioned publication exist at the electrodeelement edges and have an undesired effect by being positioned oppositeand therefore, so to speak, "seeing" adjacent electrode elements of adifferent potential. The invention thereby provides a precipitator inwhich the attainable voltage between adjacent electrode elements islimited primarily only by the electrical properties of the dust whichhas precipitated on the electrode elements and which itself can beregarded as field-concentrating formations located on the electrodeelement surface inwardly of the electrode element edges. In this manner,self-stabilisation of the voltage between the electrode elements isachieved at a higher voltage level than can be achieved according to theprior art.

BRIEF DESCRIPTION OF THE DRAWING

The single figure in the accompanying drawing is a fragmentaryperspective view of an exemplary embodiment of a precipitator accordingto the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

This precipitator may be a part of the two-stage electrostatic filterdisclosed in the above-mentioned publication, for example, but it mayalso be incorporated in other electrostatic filters.

The stream of air which carries charged particles to be precipitated onthe electrode elements of the precipitator is indicated by an arrow A.Only a portion of the precipitator is shown, namely two pairs ofelectrode elements 11 and 12, respectively, in the form of flat plates,a lateral wall 13 of insulating material, which supports the electrodemember plates at one edge thereof, i.e. a lateral edge extending in thedirection of flow of the air stream A passing through the air passageformed by the precipitator and subdivided into a large number ofsubpassages by the plates, and finally a contact strip or bar 14 ofconductive rubber or plastic material through which the plates 12 areconnected to a reference potential, preferably ground, at theirdownstream edges.

The lateral or longitudinally extending edges of the plates 11 and 12are received in groves 15 formed in the opposing sides of the side walls13 which are made of an insulating material, such as expanded plastic.

In the illustrated embodiment the precipitator plates 11 are maintainedat the required potential by being charged by the air ions which aretransported in the same direction as the air stream A and surrendertheir charges to the plates. However, it is within the scope of theinvention to maintain the plates 11 at the required potential by meansof a high-voltage source. If a high-voltage source is used, itpreferably has a high-resistance connection with the plates and isadapted to limit the charging current to very low values.

As shown in the drawing, because of the manner of charging of the plates11 by air ions, these plates extend a short distance beyond the plates12 in the upstream direction, i.e. against the direction of flow of theair stream A, so that the air stream first encounters the leading orupstream edge of the plates 11. At the downstream end of the air passagethrough the precipitator, the grounded plates 12 extend beyond theplates 11 in the downstream direction so that they are readilyaccessible for connection to ground through the contact strip 14.

Plates 11 and 12 are made of a semiconducting or dissipative material,preferably a fibrous cellulose material, such as cardboard or paper, andscreened in the above-described manner.

Thus, at their upstream or leading edge, plates 12 have a coating 16 ofan insulating material. This coating may be applied by painting or insome other suitable manner, such as by attachment of an edge strip oredge bar.

Plates 11 are provided with a corresponding screening 17 at theirdownstream or trailing edge. These plates are also screened at theirlongitudinal or lateral edges because these edges are received in andcontacted by the insulating material of the walls 13. These edges thusdo not require a separate coating of insulating material.

Thus, in accordance with the invention, those plate edges which, so tospeak, "see" the upper side or the under-side of adjacent plates arescreened such that field-concentrating formations at these edges cannotcause discharges to adjacent plates.

At their downstream edges, the grounded plates 12 are provided with aconductive coating 18 forming an electrical connection between the bodyof the plates and the contact strip 14.

We claim:
 1. An electrostatic precipitator for a two-stage electrostatic filter, comprising first and second groups of electrode elements (11,12) which are positioned side by side and spaced apart to define flow passages for air from which particles are to be precipitated, the electrode elements (11) of the first group alternating with the electrode elements (12) of the second group and being adapted to be at a potential different from that of the electrode elements of the second group, wherein the electrode elements of the second group have upstream edges positioned downstream with respect to upstream edges of the electrode elements of the first group and at least the electrode elements (11 and/or 12) of one group are made of, or coated with, a semiconducting material and wherein at least the electrode elements (12) of the second group comprise, or are provided with, a screen (16) of an insulating material at least at the upstream edge thereof. 